RS700S — Wi-Fi upload capped at ~400 Mbps in router mode only (hardware acceleration / ONT issue)

this is not a WLAN but a routing issue.... with customer support we tried to connect to the individual bands with Smart Connect disabled. Still the same.
The same problem also with my sons iphone and from my partners computer......
(1)- Asus Router connected to Zyxel ONT... no problem
(2)- RS700S as AP, no problem, same WIFI settings, full speed
(3)- RS700S as Router connected to the Zyxel ONT, upload is limited
(4)- RS700S as Router connected to the Zyxel ONT, computer connected with LAN to RS700S, full speed over LAN

So, (1) proves, the WAN connection is working with full speed with another router with Zyxel PM7300 ONT
(2) proves RS700S WIFI works with full speed always in AP mode (routing disabled)
(3) reproduces the problem, slow upload in Routing over WIFI
(4) shows, the slow routing is only affected over WIFI 

This cannot be anything but a firmware problem, i ran this with other analytics and everything points to that.